Hi Amr, Nice behavioral capture! Beauties. These are from Saudi waters? Sorry my ID skills on these Phyllidiella or Phyllidiopsis is nil. I don't see rhinophores on the critters but we can be sure that they are positioned right side to right side in the mating position of all sea slugs. In rare instances, two similar but different species attempt to mate, ref:

thanks Kevin, yeah its from Saudi water i have another pict taken also for Nudibranch mating but its not good colors i had no time to change my settings as i was running low on NDL , and ur right they will be on their right side when they start the process ,Nudibranch are one of my favourite subjects